A system has been developed for the study in vivo and in vitro of antigenic changes occurring in rat virus-induced leukemia cells under the effect of the immune response. In vivo, leukemia cells cease to express x-irradiated rats but revert to antigenic expression even after long intervals of serial antibodies. The cycles of antigenic conversion and reversion of leukemia cells will be monitored in vivo and in vitro with the use of indirect immunofluorescence using anti Gross Murine Leukemia virus antisera as well as anti p12, p30 and gp70 monospecific antisera. The antigenic modulation will be also periodically monitored by cytotoxicity tests and assays for the presence of reverse transcriptase. Histologic and electron microscopic studies will investigate the morphologic alterations that accompany the antigenic changes. The oncogenicity and the virus production of modulated leukemia cells will be assayed by cell free filtrates in susceptible newborn rats. The aim of this project is to study the mechanisms of antigenic modulation of leukemia cells and its correlations with tumor growth.